Coating composition for the manufacture of printable coated paper and board, component of the coating composition and process for the manufacture of paper and board.
The present application deals with a coating formulation for the manufacture of coated paper and board, and a selected polymeric component for the production of coating colours, including a manufacturing process for paper and board.
Paper and board are composed of individual fibers, which cause a surface roughness related to the dimensions of the fiber components. Surface roughness adversely effects printability of paper and boards. In order to reduce this surface roughness, paper and board are coated with coating colours containing pigments like coating clay, fine and ultrafine ground limestone, dispersants, converted starch polymeric binder emulsions and other additives. The coating colour can be applied in several process stages, for instance as pre and topcoat.
Ideally application of the coating mix results in complete and homogeneous coverage of the raw stock surface and with no penetration into raw stock pores. The coating colour can be applied to a web running at speeds up to 1800 m/min with a roll applicator the surplus of applied colour is metered off with a blade. Drying of the coated web is effected with available and known technologies.
The process can be integrated into the paper production process or can be carried out as a separate production stage. The viscosity and runnability of coating colours depend largely on the solids level of the mix. A high solids level can reduce drying energy consumption, however high viscosities can cause problems, like high hydraulic flow resistance in pipes, poor colour distribution in the applicator pond and excessively high blade pressure. These problems can lead to reduced production speed.
A low solids level of the coating colour results in excessive wettening of the raw stock and hence a dramatic reduction of raw stock strength and an increased number of web breaks. Also drying energy consumption increases significantly. The coating colour should also exhibit viscosity stability over extended time periods.
High solids content in conjunction with suitable and stable viscosities are sought to be achieved with dispersants and other additives. Also dispersants derived from natural raw materials are used widely. An especially effective natural dispersant is carboxymethylcellulose.
For coating formulations containing UFGL (ultrafine ground limestone) as the major pigment, the use of gelatine for this application has been described for instance in patent DE-C-195 29 661. Also dispersants on a fully synthetic basis, such as polyvinylpyrrolidone are described in the same patent.
The present invention provides a coating formulation, which allows to obtain a coated product with improved quality parameters like brightness and gloss and a further improvement of runnability, e.g. avoidance of bleeding and streaking at the blade and loss wetting of the raw stock web. The coating mix of this invention is particulary suitable for the application of UFGL, which because of its high brightness and wide spread availability gains steadily more importance compared to kaolin. However, the high percentage of fines in UFGL can cause runnability problems.
For make up of a coating composition pigments or pigment mixtures, soluble and/or dispersed binder polymers are thoroughly mixed. Soluble polymers can cause volume exclusion or depletion flocculation of dispersion and emulsion particles. As a result wet packing characteristics of the coating colour can deteriorate. Also blending of pigment slurries with different optimal dispersant requirement can cause formation of pigment aglomerates due to over or underdispersion of a specific pigment fraction. As a result water release from such coating formulations into the web during the application stage is much higher.
It was now found, that in contrast to the generally used electrostatic stabilization, steric stabilization provides high viscosity stability, both over extended time periods at constant solids and in relation to solids variations occurring as a result of the coating process.
Steric stabilization functions by formation of a polymer layer around the particle. According to Huggins and Flory, the polymer should show a high interaction potential with the solvent. The attached polymer layer prevents formation of pigment agglomerates even at a very high solids level.
The coating mix of the present invention allows rapid structure formation in the wet film and prevents penetration of pigment and binder fines into raw stock pores. Thickness variance of the metered coating film is reduced accompanied by reduced micro gloss variation and improved calenderability.
Conventional coating formulations show much more penetration of fines into the raw stock pores, which over time leads to a coarsening of the pigment particle size distribution in the application system and gradually deteriorating gloss and smoothness parameters of the coated product. Because of increased coating thickness variance, micro gloss variations of the coated surface become very objectionable.
The present inventions relates to an aqueous formulation with a solids level range from 50 to 80% by weight, comprising pigments and binders, whose viscosity at 10 secxe2x88x921 is higher than 10xe2x88x923 mPa.s, preferably above 104 mPa.s and especially preferred 105 to 108 mPa.sec. At a shear rate of 105 secxe2x88x921 viscosity of the coating composition of this invention drops to values ranging from 30 to 100 mPa.sec.
Viscosities were measured with a Stress Tec Viscometer manufactured by Reo Logica, Lund, Sweden, a Brookfield viscometer, manufactured by Brookfield Engeneering Laboratories, 240 Cushing St. Stroughton, Mass. 02072 USA and a HACAV II capillary viscometer.
The viscosity values determined according to the method of Brookfield are dependent upon spindle type and rotation velocity. The viscosity values indicated in this patent application have been determined by using spindle No. 4 and rotation velocities of 10/min or 100/min, respectively. The viscosity values were determined at temperatures of 20xc2x0 C.
Preferred are coating formulations containing kaolin and especially preferred are formulations containing UFGL. Especially preferred is UFGL with a particle size distribution of 90-99% by weight below 2 xcexcm diameter. Such pigments are used for production of high gloss printing paper grades.
The invention relates to pigment particles, which are encapsulated by a polymer shell, which prevents formation of pigment agglomerates. A preferred embodiment of the present invention contains pigment particles with a polymer shell consisting of graft polymers derived from proteins grafted with ethylenically unsaturated monomers with functional amide or amino groups.
Such polymers are strong polymeric Lewis bases (electron donors) and show a high adsorption affinity to the pigment surface. The adsorbed polymer layer stabilizes the pigment particles especially the ultra fine pigment fraction.
Examples of polymers, which can be used within the scope of this invention, are proteins like soy proteins, casein and as the preferred embodiment gelatine, which is polymer grafted with acrylamides, methacrylamides, aminoalkylacrylates, aminoalkylmethacrylates or blends of these monomers.
The polymers applied according to this invention can contain primary, secondary or tertiary amino groups. Other examples of polymers, which can be used according to this invention are proteins like soy proteins and preferably gelatin, which is grafted with lactames comprising ethylenically unsaturated groups, such as N-vinyllactames, like N-vinylcaprolactam or N-vinylpyrrolidon or with a blend of said monomers.
Especially, preferred components for the preparation of coating formulations, according to this invention are gelatines grafted with amide or amino group containing ethylenically unsaturated monomers. The weight ratio between ethylenically unsaturated monomers and gelatine can reach to a 1:1 ratio, preferably between 0.1:1 and 1:1.
These are novel compounds and are also an objective of the present invention.
Apart from A-type (acid hydrolized) gelatins, B-type gelatines (alkali hydrolized) are preferably used as backbone for the grafting process. Gelatines with Bloom values of 80 to 240 preferably between 100-160 are used.
Examples for amide group containing ethylenically unsaturated nonomers are amides of acrylic acid or of methacrylic acid, N-vinyllactemes, like N-vinylpyrrolidon and N-vinylcaprolactam.
The grafting process can be carried out in the aqueous phase by using state of the art initiators like the redox system, peroxydisulfate/sodiumdithionite. The grafting process has to be controlled carefully to avoid undesirable crosslinking reactions, which will reduce solubility of the graft polymers and can interfere with their capability to act as steric stabilizers. Excellent results are obtained, if the grafting reaction is carried out in the presence of urea or similar acting compounds like guanidine salts, dicyandiamide or melamine. These compounds improve both, solubility of the gelatine and contribute to obtain a grafted product with good solubility. The solubilizing agent like urea is preferably added to the gelatine in a 1:1 weight ratio or with a slight surplus.
Preferred are coating formulations, which contain between 0.1 to 1 pts by weight of dry graft polymer on 100 pts by weight of dry pigment, especially between 0.2 to 0.5 pph. Preferred are gelatines grafted with N-vinyllactames, like N-vinylpyrrolidone but preferably N-vinylcaprolactam.
Examples of binder and other additives are styrene-butadiene or styrene-acrylate dispersions, optical brightening agents like 4,4-diaminostilbenedisulfonic acid, preservatives, dispersants like sodiumpolyacrylate, lubricants like sodium-, ammonium- or calciumstearate, defoamers and dearators, caustic soda or ammonia for pH-adjustment and crosslinkers like urea, or melamine-formaldehydecondensates, gyloxale, glyaxale resins and dispersed or water soluble oxirane resins. The coating mix of the present invention contains water as a preferred vehicle.
Another preferred embodiment of the present invention contains between 0. 1 to 1 pph, preferably between 0.2 to 0.05 pph of an amino group containing polymer, especially polymers derived from esters of ethylenically unsaturated carboxylic acids like acrylic or methacrylic acid with amino alkylalcohols, especially the N,N dialkylsubstituted derivates.
Such polymers can be obtained by radical polymerization of monomers like dimethylaminoothylmethacrylate or -acrylate in the presence of azoinitiators. pH of the reaction mixture can be adjusted with carboylic acids, like acetic acid. Preferably the polymerization is carried out in the presence of dissolved polyvinylalcohol.
The reaction product of the polymerization forms hydrogen bonds with polyvinylalcohol. The polymer complex with polyvinylalcohol acts as a protective colloid for pigment particles.
The reaction is preferably earned out in 100 parts polyvinylalcohol with 10 to 25 parts of N,N-dimethylaminoethylmethacrylate or -acrylate in the presence of 0.05 to 0.5 parts of an azoinitiator.
Viscosity of the coating colours of the present invention as measured with a Brookfield viscometer at 100/min are below 1100 mPa.sec preferably below 900 mPa.sec at 10/min viscosities range from 2000 to 7000 mPa.s.
The coating mixes of the present invention produce particularly good results if used in conjunction with very fine coating pigments, which with conventional state of the art systems tend to form streaks at the bade, excessive micro gloss variations and uneven ink acceptance of the coated sheet.
Especially UFGL pigments, which if agglomerates are generated in the coating produce low gloss can advantageously be used as the sole or major pigment component to produce coated grades with high brightness and gloss.
Without being bound by theory it appears, that the surprising effect of the coating mix of the present invention originates from a three dimensional structure between pigment and binder dispersion particles caused by adsorptive polymer bridges, which prevents separation effects between coarser and finer particles as experienced with state of the art formulations.
The three dimensional structure is disrupted by the application of shear stress but rebuilds after shear stress has subsided. State of the art systems depend on thickening of the aqueous phase with soluble polymers only and do not show this effect.
As very low shear rates but high hydrodynamic pressures are existing in the nip of a roll applicator, the three dimensional structure prevents excessive penetration of coating mass into raw stock pores. Thixotropic formulations described previously, do not exhibit, the unique time dependent viscosity variations of the coating formulation of the present invention.
In the FIGURE the viscosity curve of the coating formulation with grafted gelatine (curve 1, example 3) is demonstrated in comparison with a state of the art coating colour, thickened with carboxymethylcellulose (curve 2, example 1).
In this example, a significant higher viscosity at low shear rates is obtained for the new coating formulation. At high shear rates of 106 secxe2x88x921, which simulate blade action to an extent, much lower viscosities are obtained with the novel coating formulation.
The dense packing of the sediment layer, which forms at the raw stock/coating wet film interface with the coating mix of the invention reduces water release from the applied coating layer into the raw stock and provides both good coverage and excellent runnability at the blade.
To determine the water retention of a coating formulation coating is applied onto a filter paper placed between two electrones. With an applied voltage of 4.5 V, the time is recorded until a current of 1 mA is reached.
A more advanced method to determine water release and penetration characteristics of a given raw stock coating mix combination is the Modul C Penetration Dynamic Analyzer, supplied by Mxc3xctek Analytic GmbH in Herrsching. The method is described in detail in xe2x80x9cWochenblatt fxc3xcr Papierfabrikationxe2x80x9d 16, 1999, p. 1023-1027. The formulations of the present invention show a pronounced transmission maximum of a 2 MHz ultrasonic beam between 200 and 2000 msec.
The coating formulations of the present invention improve both substrate coverage and reduce water release into the raw stock. Blade pressure is reduced and runnability at the blade is improved. The improved substrate coverage obtained with the novel coating formulations lead to more homogeneous ink acceptance of the topcoated paper and a reduction of print mottle.
State of the art coating formulations depend in terms of gloss development of the coated sheet very much on the application solids level. Another advantage of the novel coating formulation is independence of gloss development from application solids.
Production related gloss variations originating from variations in solids level of the coating mix can thus be avoided. For top coating formulations, solids levels of up to 73% can be run without disturbing rheological streaks or bleeding at the blade. Calender pressures can be reduced. The optimal pigment packing results in high gloss levels and higher brightness of the coated sheet, higher printed gloss and uniform ink acceptance.